Numerous heat transfer systems for providing heating, ventilation, and air conditioning (“HVAC”) are known.
HVAC systems are often used to heat, ventilate, and air condition occupied, as well as mechanical, spaces in various types of buildings. For example, such systems may be implemented to serve a residential or commercial low-rise and high-rise, healthcare, and government buildings. Examples of mechanical spaces that may be served (i.e. heated and/or cooled) by such HVAC systems may include electronics rooms, such as computer server rooms, and may include mechanical rooms, such as boiler rooms and pumping stations.
For example, U.S. Publication No. 2011/0265502 A1 (“Maxwell”) describes a high-efficiency heat pump that includes a frame, as well as a first circuit, a first compressor, a condenser heat exchanger, a first electronic expansion valve, an evaporator heat exchanger, and a controller.
In an aspect, the systems described in Maxwell may include a relatively large number of components. Generally, the larger the number of components a system has, the more expensive that system may be to manufacture, install, and maintain.
In another aspect, such systems may be less suitable, and possibly not suitable, for some applications requiring such systems to satisfy some particular heating and cooling load profiles. In one particular example, such systems may be less suitable for serving spaces that have high heating and/or cooling loads for relatively short periods of time, as well as relatively low heating and cooling loads for other relatively longer periods of time.
Such heating and cooling load profiles may cause such existing systems to experience relatively high cycling of compressors (that is, compressors switching between operating and being shut off), for example, in order to satisfy part heating and part cooling loads.
In an aspect, relatively higher compressor cycling may result in lower operating efficiencies of the systems, higher electrical loads, and reduced life of the compressors. In other aspects, relatively higher compressor cycling may result in higher operating noise levels and tonal sound changes that may be audible to and irritating to occupants of spaces served by systems comprising these compressors. In yet another aspect, higher compressor cycling may result in reduced compressor lifespans.
Various add-ons and options for refrigerant compressors, such as hot gas bypass systems, Variable Frequency Drives for slowing down a refrigerant compressor at part loads, and other add-ons, may be used for refrigerant compressors in existing HVAC systems to reduce cycling of those compressors or to increase their operating efficiencies. However, there are also drawbacks to implementing such add-ons.
For example, some add-ons may permit a refrigerant compressor in a given existing HVAC system to run at part loads (as opposed to being locked out of operation due to low refrigerant pressure), but may cause that compressor to run less efficiently. Other add-ons may increase a refrigerant compressor's efficiency in some operating conditions, but may add to the cost of manufacturing, implementing, maintaining, and eventually replacing that compressor and the HVAC system in which it may be implemented. In yet another aspect, refrigerant compressors implemented with such add-ons and options may nonetheless operate most efficiently at their design operating conditions (for example, at cooling loads that are near the nominal capacity of the compressors).
Some HVAC systems have been implemented in attempts to mitigate some of the mentioned drawbacks. However, such systems may also have drawbacks. An example of such a system is commonly referred to as a Variable Refrigerant Flow (“VRF”) system.
For example, U.S. Publication No. 2013/0091874 (“Sillato et al.”) describes a VRF system having a compressor and one or a plurality of evaporators. The suction at one or the plurality of evaporators for the input to the compressor is monitored and generally corresponds to the minimum pressure of the refrigerant. The pressure is associated with a temperature and is controlled to always be above the dew point temperature of the room served by that system.
VRF systems may include indoor VRF units to air condition spaces, and outdoor VRF units to serve the indoor VRF units. VRF systems use refrigerant tubing to connect the indoor VRF units with the outdoor units, and require the compressor(s) of the systems to run in order to provide heating or cooling. Where a VRF system is implemented to, for example, air condition offices in a commercial office building, the system may include a relatively large amount of refrigerant piping.
In one aspect, refrigerant piping may be expensive to manufacture, purchase, install and maintain, relative to, for example, piping for water or glycol. The relatively large costs may be associated with the fact that refrigerant systems operate at relatively large (for example, in comparison to hydronic heating and air conditioning systems) refrigerant pressures requiring relatively stronger construction of the refrigerant piping, which, in turn, may result in larger manufacturing and installation costs.
For example, a typical refrigeration system may operate at about 600 pounds per square inch (“PSI”) (about 4137 kilopascals, or “kPa”) refrigerant pressures. In comparison, a typical water or glycol piping system may operate at, for example, 50 PSI (about 345 kPa).
In another aspect, in many jurisdictions installation and testing of refrigerant piping may require specialized technicians.
In yet another aspect, where a leak develops in refrigerant piping, the leak may be relatively difficult and expensive to identify and repair, compared to, for example, water or glycol leaks in water or glycol piping. In another aspect, even a relatively small refrigerant piping leak may cause sufficient leakage of the refrigerant to render a VRF system inoperable due to a loss of the refrigerant in a relatively short period of time.